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Stretching between British Columbia and Alberta, the Canadian Rockies aren’t just astonishingly beautiful; they also offer one of the best places to find hybrid birds.

About 12,000 years ago, the Rockies, along with most of Canada, were covered in thick sheets of glacial ice. As a result, species summering at the foot of the mountains were once isolated by frozen stretches of land. But when the ice melted about 10,000 years ago, it was like lifting a curfew. Birds used their new freedom to venture out of their mating boundaries.

Darren Irwin, a professor of zoology at the University of British Columbia (UBC) who studies the bonkers breeding activity in the region, likes to refer to the Rockies as “the great Canadian suture zone.” “[It’s] where you have different biotic groups that have kind of met recently," he says. "And by recently I mean within the last 10,000 years."

Over the course of thousands of generations, pockets of birds developed unique traits to aid in their survival in the region. But they didn’t quite form distinct species, a process Irwin says usually takes around a million years. Instead, they blended with each other, filling forests with mixes of yellow- and red-shafted Northern Flickers, Mourning and MacGillivray’s Warblers, and Myrtle and ����APP’s Warblers. Fast-forward to today, and the region is one of the richest hybrid zones on the continent, on par with combo-rich areas in the Amazon, Central Siberia, and the Great Plains.

Delving into the hybrids of the Canadian Rockies has helped solve a few species mysteries. For instance, the ����APP and Myrtle Warbler were once considered separate species—until a hybrid population was uncovered in western Canada, forcing ornithologists to reconsider their taxonomy. Now, Irwin and his former student David Toews are trying to pinpoint how the two Yellow-rumped Warbler subtypes pass on their traits to offspring.

Generally, interbreeding is thought to drain a species’ genetic resources by shortening lifespans and limiting population growth. But Irwin and Toews’ research shows that this view isn’t necessarily true. They found that hybrid zones can help parent species swap positive traits and eliminate unhelpful genes. Over generations, a “bad” gene might gradually disappear in the zone, while a “good” gene may eventually find its way to new individuals and populations, allowing them be more successful. “I think of it like a filter,” Toews, who’s now at the Cornell Lab of Ornithology, says.

Of course, none of this would be possible without the unique geography of the Canadian Rockies. From alpine forests to lowland prairies, the region offers spacious, diverse habitats for species to spread out in, Toews says. These niches can further promote the development of new characteristics and, in time, maybe new species.

So, if you ever get to hike the Canadian Rockies, keep in mind that there may be more to the birds you see flitting through the pines; they might just be local-specialty hybrids.